Generally to start a cold engine, a richer fuel mixture is required, which results in a majority of the exhaust emissions generated. Therefore it is desirable to design a cylinder head that can tolerate relatively lean fuel mixtures in a cold engine. It is also desirable to design an engine to accommodate relatively high levels of exhaust gas recirculant (EGR) for reducing emissions, without sacrificing a loss in combustion stability or power output. Such an increased tolerance for air dilution may be accomplished by generating charge turbulence. Research has shown that an approximately even combination of swirl, circular flow about the cylinder axis, and tumble, circular flow about an axis transverse to the cylinder axis, leads to an optimal flow condition by maximizing the dilution tolerance per total angular motion imparted to the charge. A four valve per cylinder engine tends to provide primarily tumble because the two intake valves are located symmetrically about the cylinder center, whereas a two valve per cylinder engine inherently provides swirl since the single intake valve is offset from the center of the cylinder. Therefore the two engine configurations have different hurdles to overcome to generate the desired angular motion.
Mechanisms such as variable valve and variable port actuation may generate the charge motion desired, but at increased expense. Fixed geometry structure may also generate motion, but may lead to a loss in engine power by restricting air flow. The need remains for a fixed geometry configuration that provides the proper angular motion while not impeding air flow, and therefore power output.